1. Field of the Invention
The invention relates to an improved process for preparing methyl acrylate or mixtures of the latter with acrylic acid.
2. Description of the Prior Art
The methyl ester of acrylic acid is a valuable product which is employed more particularly in preparing polymeric products of wide use. Acrylic acid is employed to a great extent in actual practice, too.
Various processes are known in the art for preparing the esters of acrylic acid, such as those based on hydrolysis of acrylonitrile or on the reaction of acetylene, carbon monoxide and an alcohol in the presence of a nickel carbonyl complex.
According to a widely used industrial process, propylene is oxidized to acrolein on suitable oxidation catalysts and the resulting acrolein is catalytically oxidized to acrylic acid.
Finally, the acid is esterified by reaction with a lower aliphatic alcohol in a further reaction step.
The above described process is rather complex owing to the multiple reaction steps which necessitate a number of onerous purifications and recyclings. Moreover, the overall yield of acrylic ester is relatively low.
According to a further process known in the art, propylene is directly oxidized to acrylic acid on suitable catalysts in one reaction step. With this process the conversion of propylene to acrylic acid is generally very low and large quantities of unsaturated aldehyde mixed with the unsaturated carboxylic acid are moreover produced.
It is therefore necessary to separate the unsaturated aldehyde and unaltered propylene from the reaction products, as well as to purify and recycle such products with the inherent disadvantages.
According to our prior U.S. patent application Ser. No. 469,446 filed May 13, 1974, now abandoned, the methyl ester of the acrylic acid is prepared by contacting a gaseous flow comprising acrolein, oxygen and methanol with special catalysts. This is a considerable simplification in the process for preparing methyl acrylate.
Moreover, according to the cited application, acrylic acid can be produced together with methyl acrylate.
The process is so flexible that adjustment of the ratio of acrylic acid to its corresponding methyl ester in the reaction products is made possible simply by varying the acrolein/methanol ratio in the gaseous feed to the catalyst.
Suitable catalysts for such conversion are oxides of molybdenum, vanadium and/or tungsten, possibly combined with further metal oxides.